The present invention relates to a wearable time-bracketed video authentication system.
Recorded evidence, such as videotapes of crime scenes, police interviews, and actions performed in fulfillment of a contract, statute, ordinance or treaty, are ordinarily authenticated by the testimony of witnesses present when the recording was made, supplemented by an unbroken chain of custody between the time the recording was made and the time it is used in evidence. If the chain of custody were broken, for example if the recording were borrowed for a few days by an unscrupulous person, modern digital signal manipulation techniques would make it possible to falsify the evidence, for example by altering an internal time stamp or by adding or removing scenes, objects, sounds, data or the like.
This problem has been dealt with by a technique known as time stamping. For example, U.S. Pat. No. Re. 34,954 to Haber, et al., discloses a time-stamping technique in which a digital document is hashed using a one-way hash function to form a hash digest. The digest (a binary number) is sent to a trusted certification agency, which assigns a time stamp and uses a public key cryptosystem to encrypt the combination of the digest and time stamp. By decrypting this combination at a later time, using the agency's public key, it is provable that the agency in fact combined the hash digest with the time stamp. Since the agency is trusted, it is inferred that the hash digest was submitted to the agency at the time corresponding to the time stamp. By the nature of one way hash functions, no other document can plausibly have the same hash digest value.
The Haber method authenticates the fact that a given digital document was submitted to the agency at a certain time. It says nothing about the relationship between the document and a set of circumstances in the physical world that the document may purport to faithfully represent. For example, a scene could be staged and videotaped at time A, authenticated with a later time stamp B, and falsely claimed to be a representation of events that occurred at any desired time prior to time B. Alternatively, the videotape could be assembled from portions made at different times (all prior to time B). Thus, the Haber method provides no way to bracket the time of creation of the video since only a latest time of creation (i.e., time B) is authenticated.
The importance of knowing that a certain event did not take place before its purported time is illustrated by a form of insurance fraud. A motorist involved in an accident in which he was not at fault might take a picture of his damaged car and send it to his insurance company, concealing the fact that most of the damage occurred in an earlier, unreported accident in which he was at fault.
Another approach to authenticating recordings is disclosed in U.S. Pat. No. 4,922,339 to Stout, et al., which describes a system for visual surveillance and documentation which comprises one or more cameras to visually record an event and a transducer that derives (non-visual) information about the event. The visual signal and the information signal are integrated to form a third signal that is recorded or displayed. This is asserted to be done in such a way that neither the visual nor the information portion can be altered independently of the other portion. An example shows a truck being weighed and the weight measurement being combined with a visual image of the truck. Time stamping is optional.
While Stout claims to provide an unalterable record linking a video image to some measured information about the physical world, possibly including a time stamp, Stout provides no means of ensuring unalterability. Instead, Stout relies on the combining of two signals to form one signal using a commercially available device but does not consider the possibility that the combined signal could be decomposed and a new combined signal reconstructed in which one of the components has been altered. Even if a time stamp is used, no authentication of the time stamp is provided by Stout. Time stamp authentication is of course known in the art as evidenced by Haber but even if applied to the Stout system, such authentication only provides a latest time at which the recording could have been made.